Add the following code to Admin Panel > Theme > Custom CSS.
The first group is the search field as it displays when inactive.
The second group is when it is in focus.
background-color and border-color should be self-explanatory. color refers to the color of any text that is entered by the user.
Adjust the colors as you see fit.
form[action="kb.php"] #basic_search .input.attached input, form[action="orgs.php"] .input.attached input, form[action="users.php"] .input.attached input, form[action="tasks.php"] .input.attached input, form[action="tickets.php"] .input.attached input {
background-color: #ddd;
}
form[action="kb.php"] #basic_search .input.attached input:focus,
form[action="orgs.php"] .input.attached input:focus,
form[action="users.php"] .input.attached input:focus,
form[action="tasks.php"] .input.attached input:focus,
form[action="tickets.php"] .input.attached input:focus {
background-color: #ccc;
border-color: #eee;
color: #000;
}